Head portion of rock drill



FIG. 1 is an overall, front view of a rock drill having a head portion bearing the design;

FIG. 2 is a right view of the head portion of the rock drill of FIG. 1;

FIG. 3 is a left view of the head portion of the rock drill of FIG. 1;

FIG. 4 is a front view of the head portion of the rock drill of FIG. 1;

FIG. 5 is a back view of the head portion of the rock drill of FIG. 1;

FIG. 6 is a bottom view of the head portion of the rock drill of FIG. 1; and,

FIG. 7 is a top view of the head portion of the rock drill of FIG. 1.

The portions inside the broken-line frames form no part of the claimed design. 

The ornamental design for a head portion of a rock drill, as shown and described. 